Short answer: Nothing.

Longer answer: Nothing unless you want to set this guy an ultimatum.

I'm not joking. You could tell the guy that you're not willing to wait for him and he has to decide between you and his ex-wife/ex-girlfriend/ex-whatever, but the chances are good that that will backfire and you'll lose him. If you don't want to do that - and that's prudent, because, like I said, setting an ultimatum has a high chance of failing - then there's nothing else you can do until he makes his decision. It sucks, sure, but that's life.